1. Technical Field of the Invention
The present invention relates to a thermal printhead for forming an image in a recording medium by heating of a heating element, and more particularly to a technology for protecting a drive IC which drives the heating element of the thermal printhead.
2. Background Art
A thermal printhead comprises a substrate, a plurality of heating elements arranged in a row on the substrate, a plurality of drive IC's for driving the heating elements, and a sealing portion for sealing the drive IC's.
Each of the heating elements is formed by electrically dividing a heating resistor formed along an edge of the substrate. The drive IC's is mounted on the substrate and are mutually connected via a predetermined wiring formed on the substrate and a plurality of wires. Each of the drive IC's has a predetermined number of the heating elements allocated, and each of the drive IC's selects a heated state or a non-heated state for each of the heating elements allocated thereto. Therefore, by having each of the drive IC's drive to heat selected heating elements, an image is outputted in a recording medium which is being transferred on the heating elements.
The sealing portion is made of resin, and is formed on the substrate to cover the drive IC's and the accompanying wires. Purposes of providing the sealing portion include to protect the drive IC's and the wires from external forces, to insulate the drive IC's and the wires from moisture and chlorine attacks, and to shield from external light. Therefore, the sealing portion must be high not only in terms of strength and moisture resistance but also in terms of light shielding. In order to meet these requirements, the sealing portion is formed by pouring and setting a resin component such as an epoxy resin, containing a black pigment such as carbon black, and/or a filler such as silica, depending on a condition.
A thermosetting resin, such as the epoxy resin, attains a high level of hardness once becoming solid. The hardness increases if the filler is added. Further, the addition of the filler reduces shrinkage of the thermosetting resin at the time of hardening. However, a large amount of filler added will make the thermosetting resin brittle, decreasing the ability to protect the drive IC's and the wires from external forces. In view of the brittleness, the amount of the filler to be added to the thermosetting resin for the sealing portion must be limited within a certain range, which means that the shrinkage of the thermosetting resin to or beyond a certain extent must be accepted. As a result, components directly contacting the thermosetting resin, including the drive IC's, wires, substrate surface and the wiring formed on the substrate, comes under a substantial stress caused by a shrinking force exerted when the thermosetting resin hardens. Further, since the thermosetting resin sets as shrunken, the stress acting on the drive IC's and other components will remain un-released within the formed sealing portion.
If the filler added to the sealing portion has a large average grain size, it is more likely that the filler grains are pressed onto the drive IC's and other components, and at a greater force. Conventionally, the filler having a relatively large average grain size of about 15 .mu.m is used in the sealing portion, and this leads to a problem of damage in the drive IC's and wirings caused by the residual stress.
The damage to the drive IC's and the wiring may be avoided if the conventionally used filler, having a relatively large grain size, is thermally melted to round sharp edges of the filler grains. However, even with this operation, because of the large average size of the filler grains, the filler grains can be split or cracked by the shrinking force or the residual stress of the thermosetting resin to have sharp edges again. This problem is more serious in such a filler as silica which is a material susceptible to splitting and cracking. If the phenomenon described as above develops, it becomes impossible to properly avoid damage to the drive IC's, wirings and so on caused by the residual stress in the sealing portion.